This invention relates in general to a novel resistor composition and to a method of producing such composition. Nickel-chromium alloys are extensively used as the resistive medium in discrete film resistors and in hybrid circuitry. These alloys are employed not only because of their high resistivity but also because they exhibit acceptable stability at elevated temperatures and because they can be deposited with a low temperature coefficient of resistance (TCR). They do not necessarily have a low coefficient of resistance unless properly deposited.
Stability may be defined as the change in resistance of a resistor composition with time. TCR may be defined as the reversible fractional change in resistance of a resistor composition with temperature.
While nickel-chromium alloys are acceptable for many purposes, over the years, the requirements for premium quality, precision resistors have been gradually tightened. One requirement which modern resistors for specialized applications are required to meet is that they exhibit a stability defined as being less than 0.5 percent change in resistance after they have withstood 2,000 hours at 175.degree. C. in air. Moreover, in addition to this stability requirement, it is desirable that modern resistors for specialized applications have a temperature coefficient of resistance, or TCR, which meets a minimum standard of 0.+-.(25.times.10.sup.-6).degree.C..sup.-1. Those skilled in the art will appreciate that such a TCR standard may also be stated as .+-.25 ppm .degree.C./.sup.-1. Such a standard has been incorporated into the current military specifications namely MIL 55182.
With standard binary nickel-chromium alloys, stability within the above-range, i.e., less than 0.5 percent change in resistance after 2,000 hours, may be obtained with a high percentage of nickel in the composition such as, for example, 80 percent nickel, 20 percent chromium by weight. However, with such a resistor composition, TCR is excessive, usually in the range of several hundred ppm .degree.C..sup.-1. Increasing the chromium concentrations drives the TCR closer to 0, but at the expense of stability.
It is a specific object of the present invention, to provide novel resistor compositions which meet the foregoing stability requirements and yet which exhibit less than .+-.25 ppm .degree.C. TCR, and thus which fall within the aforementioned military specification.
Moreover, it is a further object of the invention to provide such resistor compositions and a method of producing the same which is reproducible such that predictable resistors may be obtained within the above standards on a production basis.